I remember my school used to use turnitin.com and this is back in 2006. Basically from what i remember, you turn in your paper to their website and the computer scans it to see if there are other essays like that on the web! They are trying to see if you wrote the essay or you used someone else's ore just got it online. One problem i had with that system was once you turn your paper in, now your paper will be added to the system so YOU can't even use your own paper for yourself anymore for the future because it will flag it as this paper has been written before by someone and it's not original. I had that problem where i submitted my paper from my previous class to my new professor and it flagged it and I had to prove that it was actually my own work that i have turn it in before!
Good luck!